Like most modern American beef breeds, the Red Angus breed had its beginning in Europe. In the eighth-century, according to some authorities, hardy Norsemen raiding the coasts of England and Scotland brought with them small, dun-colored hornless cattle, which interbred with black native Celtic cattle of inland Scotland.

A naturally polled black breed was produced, which roughly corresponded to the black Aberdeen-Angus of today, although it was a considerably smaller-bodied animal. The polled characteristic was very slow to spread inland, and for almost 1,000 years was confined principally to the coastal areas of England and Scotland.

Eric L.C. Pentecost, the noted English breeder of Red Angus cattle, offers a specific and logical explanation for the introduction of the red coloration into the Aberdeen-Angus breed. In the 18th century, the black Scottish cattle were too light to provide sufficiently large draught oxen, so larger English longhorns, predominantly red in color, were brought in and crossed with the black native polled breed. The resultant offspring were all black polled animals, since black is a dominant color, and red a recessive one. However, all carried the red gene. Subsequent interbreeding produced an average of one red calf in four, in accordance with Mendel's law of heredity.
http://www.redangus.org